It can be important to the structure of your exercise regimen to be aware of which activities require more or less oxygen uptake. Oxygen uptake essentially is a measure of how much oxygen, and therefore energy and exertion, your body is using during a given activity. Running is a popular exercise activity, and one of the most natural ways to vary this exercise is to change the incline that you are running on. But how would this affect oxygen uptake?
Level Running Versus Uphill Running
For the purposes of comparison, level running will be the baseline against which the alternatives can be judged. Working from there, it seems intuitive enough to think that going uphill will definitely require more oxygen uptake. As you go up you will be working against gravity and your body will be using more energy to account for this. You will not be able to flow into a stride like you could with level running--running heel to toe in the most efficient way. You will also need to push up to reach new elevations, making use of your calf muscles and quadriceps towards this end. The bottom line is that your body works harder and oxygen uptake is higher.
The interesting scenario is to consider going downhill. Our intuition might lead us to the conclusion that downhill running is easier and therefore requires less oxygen uptake. It's tempting to think that all the energy that was used to get uphill (whether by car or your own two legs) is energy you don't need to use when going downhill. Isn't going downhill just a series of effortlessly controlled free-falling?
Well it's a little trickier than that. When going downhill, you will be working with gravity, possibly too much gravity. Your body will make heavy use of your quadriceps to absorb the impact of the micro-free-falling you'll be experiencing going downhill. Your posture will change and other muscle groups will be incorporated towards maintaining balance. And much like going uphill, you won't be able to run heel to toe and flow into an efficient stride. All this points to using more energy and having higher oxygen uptake.
The misleading bit of information that your brain receives is that you will be covering more ground much faster, and you therefore conclude you will have a shorter workout. But the more important angle to consider is the amount of work being done per a certain time period, which directly translates to oxygen uptake. You will cover ground faster, true, but you'll still be running more strenuously for the same amount of time.
There is one consideration that must be taken into account when comparing downhill and level running and its effect on oxygen uptake. There is a point after which downhill running requires more energy and this has to do with the slope of the incline. For minor inclines, your quadriceps will not have to work too hard to absorb the shock of free-falling. You posture will not change much and your calf muscles will only need to perform minimal work. But when the decline is steep enough, those previously mentioned muscles will be working overtime to maintain balance and absorb shock. At that point and beyond, you can be sure that level-running will have lower oxygen uptake than that more strenuous downhill experience.